A year before 800,000 Rwandans were systematically murdered in a genocidal purge of Tutsis and moderate Hutus, Colonel Theoneste Bagosora stormed out of peace talks to resolve the central African country's ethnic tensions and warned he was going to "prepare the Apocalypse".

The extent to which the then senior official in Rwanda's extremist Hutu regime was qualified to deliver such an accurate forecast of the genocide that unfolded in 1994 was confirmed yesterday when Bagasora, now 67, was convicted by a UN court of masterminding the wholesale slaughter of his compatriots in the name of racial purity.

He is the first member of what was then Rwanda's governing elite to be found guilty by the International Criminal Tribunal for Rwanda (ICTR), based in Tanzania, of organising the killings, mostly committed by the Hutu-dominated army and machete-wielding members of the Interahamwe, a Hutu militia.

Some six years after Bagosora first went on trial, the tribunal's judges ruled that the chief of staff in Rwanda's Defence Ministry was jointly responsible for forming, training and arming the Interahamwe, who were unleashed when a plane carrying the country's Hutu president, Juvenal Habyarimana, was shot down on 6 April 1994.

Bagosora, who consistently denied involvement in the plot to wipe out Rwanda's Tutsis, was also found to be responsible for drawing up target lists of prominent Tutsis and moderate Hutus whose homes were targetted.

General Romeo Dallaire, the Canadian soldier who headed the UN peacekeeping mission during the genocide, described Bagosora as the "kingpin" of the killings, telling the court how he calmly presided over the start of the campaign in the hours after President Habyarimana's death.

Over 100 days of horror, the slaughter spread from Rwanda's capital, Kigali, to every corner of the verdant but tiny African country. Many victims were stopped at Interahamwe or army roadblocks and murdered on the spot after their Tutsi status was established from their identity card.

In a judgment read to a packed but silent courtroom in Arusha, the tribunal said: "Colonel Bagosora is guilty of genocide and crimes against humanity and war crimes." Bagosora, who showed no emotion as the verdict was delivered, was sentenced to life imprisonment.

The ruling was greeted with satisfaction by survivors and campaigners who have warned that dozens of senior perpetrators or genocidaires remain at large, many living under new identities abroad. The High Court in London will today finish hearing an appeal by four Rwandans accused of involvement in the genocide who were to be extradited to Rwanda after being found living new lives in Britain.

Responsibility for the firing of two missiles which blew up President Habyarimana's plane over Kigali remains a source of bitter dispute. A French investigating magistrate alleged four years ago that it was done on the orders of Paul Kagame, the present Rwandan President who was the Tutsi rebel leader, at the time. Mr Kagame angrily rejected the claim over which he temporarily severed diplomatic relations with France. What is not in doubt is that the assassination was the catalyst for the genocide.

The tribunal heard evidence that later on the night of 6 April, Bagosora presided over a military "crisis committee" meeting followed by the murder of the Rwandan prime minister and the beginning of the slaughter by members of the Interahamwe and elite presidential guard. General Dallaire, who remained in Rwanda throughout the killing but was banned by the UN from intervening with military force, said: "I concluded [Bagosora] was the kingpin... What I found incredible was I had never found someone so calm and so at ease with what was going on."

Before giving evidence to the ICTR, the retired general said his last encounter with Bagosora had been a chance encounter in a Kigali hotel in 1994 when the Hutu official waved a pistol at him and told him the next time they met he would kill him.

Bagosora, who was arrested in Cameroon in 1997, was also found guilty of being responsible for the murder of 10 Belgian peacekeepers in the early hours of the genocide, an act which prompted the withdrawal of the Belgian contingent from the lightly armed UN force. Gerard Prunier, an eminent French historian of the genocide, wrote: "It seems that, inasmuch as there was a general organiser of the whole operation, this distinction has to go to Colonel Theoneste Bagosora."

During his trial, evidence emerged that the colonel helped draft a document for circulation within the Hutu-dominated army describing Tutsis as "the principal enemy". Rwanda had long been racked by ethnic tension between the Tutsis, a minority seen as being favoured by the country's Belgian colonial rulers, and Hutus.

It was during peace talks with Mr Kagame in Tanzania in 1993 that Bagosora said he was returning to Rwanda to "prepare for the apocalypse". The colonel was alleged to have been a leading member of the Akazu, the inner circle of Hutu extremists close to President Habyarimana.

In his defence, lawyers for Bagosora tried to prove the mass slaughter of Tutsis was not organised and therefore did not meet the definition of genocide. Bagosora, whose own barrister described him as having a "very cold attitude" told the court: "I never killed anybody, nor did I give orders to kill."

The guilty verdict will be seen as further evidence of success for the ICTR. The court has heard 42 genocide cases and acquitted six defendants but has until the end of next year to deliver verdicts in the outstanding 38 cases. The UN General Assembly is discussing whether to extend its mandate.

War crimes: The monsters of Rwanda

Jean Kambanda Former prime minister

Kambanda – the prime minister at the time – was the first person to be sentenced for crimes against humanity and genocide by an international court. He was jailed for life by the tribunal in September 1998. His crimes included distribution of weapons knowing they would be used in the genocide, and ordering roadblocks to round up the Tutsis.

Jean-Paul Akayesu Former mayor

Although he was convicted after Kambanda, the genocide prosecution of the former mayor of Taba on nine counts was the world's first. He was accused of torture and initiating rape and murder, but Akayesu claimed he was unable to prevent the killings. He was sentenced to three life terms in October 1998, plus 80 years for other offence.

Ferdinand Nahimana Co-founder of Radio Mille Collines

He was on trial with newspaper editor Hassan Ngeze and Jean-Bosco Baraya-gwiza, a senior radio executive at RTLM. The tribunal found them guilty in December 2003 of committing war crimes and persecution and inciting genocide. Nahimana and Ngeze were sentenced to life (reduced to 30 and 35 years respectively), and Barayagwiza got 35 years.

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